A number of different methods have been used to hold a lens in place during the surfacing or finishing process. One method commonly used at present makes use of a low-melting-temperature alloy to form or attach a "block" to a lens. The block is adapted to be held by the surfacing or finishing machine. One common low-melting-point alloy contains a mixture of bismuth, tin, lead, cadmium, indium, and antimony. When these materials are combined correctly, the alloy melts at a temperature considerably lower than any one of its component elements would melt by itself (thus, neither causing plastic lenses to melt nor glass lenses to crack). For example, one alloy suitable for use with either plastic or glass lenses melts at 47.degree. C. and is made from the following combination of metals: 45% bismuth; 23% lead; 8% tin; 5% cadmium; and 19% indium. If desired, a single-sided adhesive tape or other lens coating may be used to achieve better alloy adherence (e.g., the non-adhesive surface of the adhesive tape may provide a more favorable surface for the alloy to adhere to than the uncoated lens surface). Blockers that use metal alloy either inject molten alloy between the semifinished lens and the block, or mold a block fully and completely from the alloy material. Both types of blockers have a melting pot that is thermostatically regulated, and a heated feeding tube. Ideally, for plastic lenses, the alloy temperature is kept just above its melting point until it fills in the area between the lens and lens block. For example, the 47.degree. C. alloy used for plastic, polycarbonate and polyurethane lenses should preferably be kept at 52.degree. C., or even lower if possible. For plastic lenses, the alloy is preferably cooled as rapidly as possible so that plastic lenses will not form surface indentations. For glass lenses the alloy temperature may be maintained at higher temperatures (e.g., 74-77.degree. C.).
An alternative approach to blocking lenses is disclosed in U.S. Reissue Pat. No. 31,897 by Johnson and involves the use of a relatively thick (approximately 0.8 mm) double-sided adhesive blocking pad which is disposed between the lens and a block. The intermediate pad serves to provide a flexible interface between the lens blank and the rigid block, which is said to permit firm bonding even when the curvatures of the lens blank and the rigid block do not match. Unfortunately, relatively thick adhesive pads of this type are often quite expensive and may be prone to shear displacement during the lens processing operation. This shear displacement (up to approximately 2.degree. to 10.degree.) can contribute to inaccuracies and errors in the prescription of the lens.
A compliant lens block is described in U.S. Pat. No. 4,925,518 (Wasserman et al.). The compliant lens block is described as being attached to the lens blank, for example, using either a pre-applied adhesive coating or a thick double-sided foam adhesive pad. Unfortunately, blocks which are precoated with adhesive can suffer from the problem of adhesive transfer to the lens upon removal of the lens from the block. This necessitates cleaning of the lens to remove the transferred adhesive (e.g., using solvents or detergents). Also unfortunately, thick double-sided foam adhesive pads suffer from the previously mentioned disadvantages of expense and shear displacement. In addition, thick double-sided foam adhesive pads oftentimes tend to remain attached to the lens blank upon deblocking. This necessitates an extra step of removing the foam pad from the lens blank. Alternatively, U.S. Pat. No. 4,925,518 discloses a one-sided, pressure-sensitive sheet material which is affixed to the block structure by application of a heat sensitive glue or curing agent. This process is expected to be quite expensive given the separate bonding step which is contemplated.